First three OFDM symbols in a sub-frame are occupied by a traditional Physical Downlink Control Channel (PDCCH), and the entire bandwidth of a system is occupied by these three OFDM symbols. For traditional PDCCH signaling, channel estimation is performed with a Common Reference Signal (CRS).
It is proposed in the 3GPP R10 relay standard to embed a PDCCH into a Physical Downlink Shared Channel (PDSCH) through Frequency Division Multiplexing (FDM) manner, and the PDCCH embedded into the PDSCH is also referred to as an R-PDCCH.
As compared with an R-PDCCH, a PDCCH based upon user equipment pre-coding, i.e., a U-PDCCH, has the following features:
The link quality between a base station and a user equipment may be not so strong as that between the base station and a relay base station, and therefore how to guarantee a BLock Error Rate (BLER) of U-PDCCH detection is an important issue.
The number of UEs number may far exceed the number of RNBs, and U-PDCCH allocation may be more dynamic. Therefore U-PDCCH resource allocation is another issue as compared to R-PDCCH.
Furthermore since channel pre-coding is performed on a U-PDCCH, for example, through Beam Forming (BF), and non-codebook based pre-coding is not supported for a CRS, so channel estimation on the U-PDCCH can not be performed with a CRS.